Cabled conductors containing anisotropic superconducting compounds

ABSTRACT

A cabled conductor comprises a plurality of transposed strands each comprising one or more preferably twisted filaments preferably surrounded or supported by a matrix material and comprising textured anisotropic superconducting compounds which have crystallographic grain alignment that is substantially unidirectional and independent of the rotational orientation of the strands and filaments in the cabled conductors. The cabled conductor is made by forming a plurality of suitable composite strands, forming a cabled intermediate from the strands by transposing them about the longitudinal axis of the conductor at a preselected strand lay pitch, and, texturing the strands in one or more steps including at least one step involving application of a texturing process with a primary component directed orthogonal to the widest longitudinal cross-section of the cabled intermediate, at least one such orthogonal texturing step occurring subsequent to said strand transposition step. In a preferred embodiment, the filament cross-section, filament twist pitch, and strand lay pitch are cooperatively selected to provide a filament transposition area which is always at least ten times the preferred direction area of a typical grain of the desired anisotropic superconducting compound. For materials requiring biaxial texture, the texturing step preferably includes application of a texturing process with a second primary component in a predetermined direction in the plane of the widest longitudinal cross-section of the conductor.

Under 35 USC §120 this application is a divisional application of U.S.Ser. No. 08/554,814, filed Nov. 7, 1995 now U.S. Pat. No. 6,247,225.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to superconducting cabled conductors and to amethod for manufacturing them.

2. Background of the Invention

The possibility of using superconductors to obtain greater efficiency inelectrical and magnetic applications has attracted considerableinterest, particularly since the discovery of superconducting materials,such as the oxide superconductors, whose structures allow them to carrysignificant currents at relatively high temperatures, above about 20Kelvin. However, to be practical outside the laboratory, most electricaland magnetic applications require flexible cabled lengths of conductormanufacturable with high packing factors at reasonable cost, in additionto high engineering current-carrying capacity.

High packing factor forms maximize performance per unit volume. Spaceconstraints and the need to handle higher overall current densities areamong the major design issues considered in most electricalapplications.

Conductors which are flexibly cabled, that is, composed of twisted,helically wound, braided or otherwise transposed bundles of mechanicallyand electrically isolated conductor strands, are desired in manyapplications, including coils, rotating machinery and long lengthcables. In comparison to monolithic conductors of comparable compositionand cross-section, cabled forms which are made from a number ofconductor strands which are substantially mechanically isolated willhave much higher flexibility. By substantially mechanically isolated ismeant that the cable strands have some ability to move independentlywithin the cable, although a degree of mechanical locking of the strandsis usually desired for stability and robustness. Flexibility increasesin proportion to the ratio between the cable cross-section and thestrand cross-section.

In low temperature metallic superconducting conductors, cables which aremade from a number of substantially electrically isolated and transposedconductor strands have been shown to have greatly reduced AC losses incomparison to monolithic conductors cf “Superconducting Magnets” byMartin Wilson (1983,1990), pp 197, 307-309, and it has been proposedthat the same relation will hold for high temperature superconductorswith more complex structures. AC losses are believed to decrease inrelation to strand cross-section, cable cross-section and twist pitch.Litz cable, a cable with multiple electrically insulated strandsassembled in a fully transposed configuration, is required for nearlyall AC applications. For DC applications, multiple uninsulated strandsmay be cabled to obtain flexibility or mechanical robustness. Thegreater the number of strands in the cable, the more pronounced theseadvantages will be. Cabling is also desirable for ease in manufacturing,since cabling processes scale more easily than monolithic manufacturingprocesses.

However, most of the superconductors, such as superconducting ceramicsof the oxide, sulfide, selenide, telluride, nitride, boron carbide oroxycarbonate types, which have shown promise for electrical and magneticapplications at relatively high temperatures are anisotropicsuperconducting compounds which require texturing in order to optimizetheir current-carrying capacity. It has not been considered feasible toform these into high packing factor, tightly transposed cableconfigurations because of the physical limitations of these materials.They typically have complex, brittle, granular structures which cannotby themselves be drawn into wires or similar forms using conventionalmetal-processing methods and do not possess the necessary mechanicalproperties to withstand cabling in continuous long lengths.Consequently, the more useful forms of high temperature superconductingconductors usually are composite structures in which the anisotropicsuperconducting compound is supported by a matrix material which addsmechanical robustness to the composite. For example, in preferredmanufacturing processes for superconducting oxide composites, such asthe well-known powder-in-tube (PIT) process or various coated conductorprocesses, the desired superconducting oxide is formed within or on asupporting matrix, typically a noble metal, by a combination of phasetransformation and oxidation reactions which occur during themanufacturing process.

Even in composite forms, the geometries in which high-performancesuperconducting articles may be successfully fabricated from thesematerials are constrained by the necessity of “texturing” thesuperconducting ceramic to achieve adequate critical current density andby the electrical anisotropy characteristic of the superconductor. Thecurrent-carrying capacity of any composite containing one of thesematerials depends significantly on the degree of crystallographicalignment, known as “texturing”, and intergrain bonding of thesuperconductor grains, induced during the composite manufacturingoperation. For example, the rare earth family of oxide superconductors,among the most promising and widely studied of the ceramicsuperconductors, require biaxial texture, a specific crystallographicalignment along two axes of each grain, to provide adequate currentcarrying performance Certain ceramic superconductors with micaceouscrystal structures, such as the two-layer and thee-layer phases of thebismuth-strontium-calcium-copper-oxide family of superconductors(Bi₂Sr₂Ca₁Cu₂O_(x), also known as BSCCO 2212, and Bi₂Sr₂Ca₂Cu₃O_(x),also known as BSCCO 2223), demonstrate high current-carrying capacitywhen uniaxially textured in the plane perpendicular to the currentcarrying direction. (Micaceous structures are characterized by highlyanisotropic, plate-like grains with well-defined slip planes andcleavage systems.) In addition, many superconducting compounds may bepartially textured by uniaxial texturing techniques. Those anisotropicsuperconducting compounds which are suitable for uniaxial texturingtechniques have been considered especially promising for electricalapplications because they can be textured by methods which are readilyscalable to long length manufacturing.

In contrast to other known conductors, such as the normal andsuperconducting metals, the current carrying capacity of well-texturedanisotropic superconducting composite articles will depend in large parton the relative orientations of their preferred direction, which isdetermined by the crystallographic alignment of their superconductinggrains, and any current flow or external magnetic field. Because oftheir crystal structure, supercurrent flows preferentially in at leastone of the directions lying within the plane normal to the c axis ofeach grain. Their critical current may be as much as an order ofmagnitude lower in their “bad” direction than in their “good” direction.Thus, an important consideration in fabricating high performance cablesfrom these materials, which is not an issue in conventional cablefabrication, is finding a way to maximize the portions of the cablewhich do have the desired orientations. For optimum current-carryingcapacity, it would be desirable to align all of the grains in the cablein parallel to one another along their relevant axes, e.g., at least thec axis for the uniaxial texturing typical of BSCCO 2212 or 2223, or atleast the c axis and either the a axis or the b axis for the biaxialtexture typical of the rare earth superconducting oxides, with eachc-axis preferably perpendicular to the longitudinal axis of the cableregardless of the relative rotational orientations of the cable strandsand filaments which contain them, but the twisting and bendingcharacteristic required for conventional cabling are not readilyadaptable to such uniform grain alignment.

Thus, an object of this invention is to provide a textured cabledconductor containing a textured anisotropic superconducting compoundhaving substantial crystallographic grain alignment which isdirectionally independent of the rotational orientations of the strandsand filaments in the cabled conductor, and a process for manufacturingsuch a cabled conductor.

Another object of the invention is to provide a novel cabled conductormanufacturing process that will allow strands of superconductingcompounds which require texturing to be used with conventionalhigh-speed cabling equipment.

Another object is to provide a mechanically stable, fully transposedhigh packing factor cabled conductor containing anisotropicsuperconducting compounds in a plurality of strands which may be highlyaspected, and a method for manufacturing such a cabled conductor.

Another object of the invention is to provide a high packing factor,well-textured cabled conductor comprising a plurality of strands eachcomprising a micaceous or semi-micaceous superconducting oxide, mostpreferably BSCCO 2223 and a method for manufacturing such a cabledconductor.

SUMMARY OF THE INVENTION

In one aspect, the invention provides a cabled conductor comprising aplurality of transposed strands each comprising one or more preferablytwisted filaments preferably surrounded or supported by a matrixmaterial and comprising textured anisotropic superconducting compoundswhich have substantial crystallographic grain alignment that issubstantially unidirectional and directionally independent of therotational orientation of the strands and filaments in the cabledconductor.

In another aspect, the invention provides a transposed cabled conductorcomprising grains of anisotropic superconducting compounds textured suchthat the crystallographic c axis alignment of each grain of thesuperconducting compound is substantially perpendicular to thelongitudinal axis of the cabled conductor independent of the rotationalorientation of the strands and filaments in the cabled conductor. Thefilaments are preferably surrounded or supported by a matrix material.

The invention may be practiced with any anisotropic superconductingcompound which requires texturing of its grains. The compounds arepreferably superconducting ceramics of the oxide, sulfide, selenide,telluride, nitride, boron carbide or oxycarbonate types, and mostpreferably superconducting oxides. By “grains” are meant polycrystallinecolonies in which the c axes are substantially coincident and the a andb axes have a multiplicity of orientations for colony-formingsuperconducting compounds, such as the micaceous superconducting oxides,and single or polycrystalline regions in which the a, b, and c axes aresubstantially coincident for those compounds which do not form colonies.

In another aspect, the invention provides method for manufacturing asuperconducting cabled conductor comprising the steps of, first, forminga plurality of composite strands, each strand comprising at least one,preferably twisted, filament having a preselected filament cross-sectionand twist pitch, preferably surrounded or supported by a matrixmaterial, and containing grains of a desired anisotropic superconductingcompound or its precursors; second, forming a cabled intermediate fromthe strands by transposing them about the longitudinal axis of theconductor at a preselected strand lay pitch, and, texturing the strandsin one or more steps including at least one step involving applicationof a texturing process with a primary component directed orthogonal tothe widest longitudinal cross-section of the cabled intermediate, and ifa precursor to the desired superconducting compound remains, at leastone thermomechanical processing step at conditions sufficient to producephase transformation in the filament material, at least one suchorthogonal texturing step occurring subsequent to said strandtransposition step; thereby forming a superconducting cabled conductorhaving a crystallographic grain alignment substantially independent ofthe rotational orientation of the strands and filaments in the cabledconductor. In a preferred embodiment, the filament cross-section,filament twist pitch, and strand lay pitch are cooperatively selected toprovide a filament transposition area which is always at least ten timesthe preferred direction area of a typical grain of the desiredanisotropic superconducting compound. For materials requiring biaxialtexture, the texturing step preferably includes application of atexturing process with a primary component in a predetermined directionin the plane of the widest longitudinal cross-section of the conductor.

In a preferred embodiment of the invention, the invention is practicedwith superconducting ceramics which are themselves micaceous orsemi-micaceous, or which have micaceous or semi-micaceous precursors,and the texturing step preferably includes non-axisymmetric deformationtexturing with a primary component of the force tensor directedorthogonal to the widest longitudinal cross-section of the cabledintermediate. By “micaceous” is meant characterized by highlyanisotropic preferred cleavage planes and slip systems, and thereforehighly anisotropic current-carrying capacity. By “semi-micaceous” ismeant characterized by a highly anisotropic grain structure but poorlydefined cleavage planes and slip systems. By “precursor” is meant anymaterial that can be converted to a desired anisotropic superconductorupon application of a suitable heat treatment. If the desiredanisotropic superconductor is an oxide superconductor, for example,precursors may include any combination of elements, metal salts, oxides,suboxides, oxide superconductors which are intermediate to the desiredoxide superconductor, or other compounds which, when reacted in thepresence of oxygen in the stability field of a desired oxidesuperconductor, produces that superconductor.

In a preferred embodiment, the desired superconducting compound issurrounded or supported by a matrix, preferably a metal. By “matrix” asthat term is used herein, is meant a material or homogeneous mixture ofmaterials which supports or binds a substance, specifically includingthe superconducting compounds or their precursors, disposed within oraround the matrix. Silver and other noble metals are the preferredmatrix materials, but alloys substantially comprising noble metals,including ODS silver, may be used.

In a preferred embodiment, each strand may be coated with a layer of aninsulating material prior to cabling so that the strands in the finishedcable will be electrically discrete. By “insulating material”, as thatterm is used herein, is meant a material with an electrical resistivityhigh in comparison to that of the matrix material used in the compositeunder the intended conditions of use.

In the most preferred embodiment of the invention, the desiredanisotropic superconducting compounds are members of the bismuth familyof superconducting oxides, and the orthogonal texturing step preferablyincludes non-axisymmetric deformation texturing with a primary componentof the force tensor directed orthogonal to the widest longitudinalcross-section of the cabled intermediate. In addition, because membersof the bismuth family tend to selectively form grains aligned with thefilament walls, at least one additional requirement is included in theprocess to overcome this tendency. In one embodiment of the invention,this requirement is that the filament cross-section, filament twistpitch, and strand lay pitch be cooperatively selected so that at eachpoint on the filament, regardless of how it is twisted, the filamentwidth in the plane of the widest longitudinal cross-section of theconductor which is always greater than, and preferably twice as large asthe filament height orthogonal to the widest longitudinal cross-sectionof the conductor. In another embodiment of the invention, thisadditional requirement is a magnetic alignment step with a primarycomponent of the field directed orthogonal to the widest longitudinalcross-section of the cabled intermediate. The magnetic alignment stepmay be done anytime after cabling. In a preferred embodiment, one ormore heat treatment steps at conditions chosen to provide crack healingin the filaments but not to melt the matrix material may be incorporatedinto the process to increase the overall strain tolerance andperformance of the cabled conductor.

By “low aspect ratio” is meant an aspect ratio less than about 2:1, andby “high aspect ratio” is meant an aspect ratio greater than or equal toabout 3:1 and preferably about 5:1.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a superconducting cabled conductor,100 in accordance with one aspect of the invention.

FIG. 2 is an expanded cross-section of the composite strand 120 shown inFIG. 1.

FIG. 3 is an expanded cross-section of the filament 200 shown in FIGS. 1and 2.

FIG. 4 is a chart of a mosaic spread typical of a well-texturedanisotropic superconducting compound.

FIG. 5 is a schematic representation of a cabling machine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1 of the drawings, a superconducting cabledconductor 100 manufactured in accordance with one embodiment of theinvention, is shown in cutaway perspective view. In FIG. 1, line a-a′defines the longitudinal axis of the conductor, line b-b′ defines themajor axis of its vertical cross-section, and line c-c′ defines theminor axis of its vertical cross-section. Line c-c′ is orthogonal to thepreferred current direction for the conductor, which flows in the a-bplane. Lines a-a′ and b-b′ together define the widest longitudinalcross-section 14 of the conductor. The cabled conductor 100 comprises aplurality of strands 110 transposed about the longitudinal axis of theconductor.

In FIG. 2, a strand 110 is shown in expanded cross-section. Each strandcomprises one or more substantially continuous filaments 200 comprisinggrains of a desired anisotropic superconducting compound. For ACapplications, it is preferred that the filaments be twisted. In thepreferred embodiment shown, each filament 200 is supported or surroundedin a matrix material 220. Referring now to FIG. 3, each grain 210 of thedesired anisotropic superconducting compound has a length 10, a width 11and a thickness 12, with the length and width being roughly the sameorder of magnitude. Typically, the preferred direction ofsuperconducting current flow is perpendicular to the thickness of thegrain. Thus, the preferred direction area 13 of the grain 210 is theproduct of its length 10 and its width 11. For the micaceous andsemi-micaceous compounds which are preferred for the operation of thisinvention, the grain thickness 12 will typically be substantially lessthan either the length 10 or width 11. For optimal current flow, it isdesirable that the grains 210 have substantially unidirectionalcrystallographic alignment with their thicknesses 12 substantiallyuniformly oriented in a single direction orthogonal to the widestlongitudinal cross-section of the cable. This direction, shown in FIG. 2as c-c′,is typically known as the crystallographic c direction. By“substantially unidirectional crystallographic alignment” is meant thatthe overall distribution of grain thickness orientations forms a normaldistribution, known as a “mosaic spread”, such as that illustrated inFIG. 4, with respect to the desired axis and the distribution has a fullwidth, half max value (f_(whm)) less than a predetermined value,typically on the order of 10-20 degrees for superconducting oxides. Thisdistribution of grain orientations has, however, not generally beenobtainable in tightly transposed superconducting conductors known in theart. As shown in FIG. 3 and in accordance with the invention, the grains210 in the cabled conductor of the present invention have been texturedto have substantially unidirectional crystallographic grain alignmentthat is substantially independent of the rotational orientation of thestrands 120 and filaments 200 in the cabled conductor 100.

Returning to FIG. 3, each filament 200 has a substantially uniformfilament cross-sectional area 20 in the plane transverse to thelongitudinal axis of the filament. At each point along its longitudinalaxis, each filament 200 will have a height 23 which is its smallestdimension in a direction parallel to the c-c′ axis and a width 24 whichis its smallest dimension in a direction perpendicular to the c-c′ axis,which will be discussed in connection with embodiments using the bismuthfamily of superconducting oxides. At each point along its longitudinalaxis, each filament 200 also has a transposition area 21 which is thecross-sectional area of the filament at that point in a planeperpendicular to the crystallographic c direction, that is, parallel tothe desired texturing direction. The transposition area 21 varies withthe rotational orientation of the filament 200 and strand 120 relativeto the desired texturing direction, being smallest at the cross-overpoints where the filament wraps in a direction perpendicular to thedesired texturing direction. It is never less than the filamentcross-sectional area 20, and may be made greater than 20 even at theperpendicular cross-over points depending on the relative strand andfilament dimensions, strand lay pitch and filament twist pitch selected.In accordance with a preferred embodiment of the invention, thetransposition area is selected to be at least ten times, and preferablyat least thirty times, the preferred direction area of a typicalsuperconducting grain to permit crystallographic grain alignment in thedesired direction at the filament cross-over points. If thetransposition area is too small at any region in the filament, a hightransport supercurrent is not assured regardless of the texturing methodselected.

The strands may include any desired anisotropic superconducting compoundwhich requires texturing and may be at least partially textured by theapplication of a uniaxial texturing orthogonal. For example,superconducting ceramics of the oxide, sulfide, selenide, telluride,nitride, boron carbide or oxycarbonate types may be used.Superconducting oxides are preferred. For example, members of the rareearth (RBCO) family of oxide superconductors; the bismuth (BSCCO) familyof oxide superconductors, the thallium (TBSCCO) family of oxidesuperconductors; or the mercury (HBSCCO) family of oxide superconductorsmay be used. The bismuth and rare earth families of oxidesuperconductors are most preferred for operation of the invention.Thallination, the addition of doping materials, including but notlimited to lead and bismuth, variations from ideal stoichiometricproportions and such other variations in the formulation of the desiredsuperconducting oxides as are well known in the art, are also within thescope and spirit of the invention. The two-layer and three-layer phasesof the bismuth-strontium-calcium-copper-oxide family of superconductors(Bi₂Sr₂Ca₁Cu₂O_(x), also known as BSCCO 2212 and Bi₂Sr₂Ca₂Cu₃O_(x), alsoknown as BSCCO 2223, respectively) are the superconducting oxides mostpreferred for the operation of the present invention.

By “matrix” as that term is used herein, is meant a material orhomogeneous mixture of materials which supports or binds a substance,specifically including the superconducting oxides or their precursors,disposed within or around the matrix. Metals are typically used. Silverand other noble metals are the preferred matrix materials, but alloyssubstantially comprising noble metals, including ODS silver, may beused. “Alloy” is used herein to mean an intimate mixture ofsubstantially metallic phases or a solid solution of two or moreelements. By “noble metal”, as that term is used herein, is meant ametal which is substantially non-reactive with respect to oxidesuperconductors and precursors and to oxygen under the expectedconditions (temperature, pressure, atmosphere) of manufacture and use.Preferred noble metals include silver (Ag), gold (Au), platinum (Pt) andpalladium (Pd). Silver and its alloys, being lowest in cost of thesematerials, are most preferred for large-scale manufacturing.

In the embodiments preferred for AC applications, each strand issurrounded with a layer of an insulating material. Becausesuperconducting composites can carry large currents at very lowvoltages, insulating materials with a broad range of electricalproperties may be used. Elemental oxides, sulfides, and nitrides,semiconductors, glasses, and intermetallics are all suitable to providethe insulating layers in the present invention. For AC applications itis preferred that each strand be coated with at least 2 micronsthickness of a suitable material during processing but in the fullyprocessed article, the thickness of the insulating material may beconsiderably lower. Preferred insulating materials include magnesiumoxide, tin oxide, boron nitride and silicon carbide. Materials which areconverted to insulating materials under the conditions for processingthe conductor may also be used.

Generally, a cabled conductor such as the one illustrated with acrystallographic grain alignment substantially independent of therotational orientation of the strands and filaments in the cabledconductor, may be manufactured in accordance with the invention by thesteps of: first, forming a plurality of composite strands, each strandcomprising at least one, preferably twisted, filament having apreselected filament cross-section and twist pitch, preferablysurrounded or supported by a matrix material, and containing grains of adesired anisotropic superconducting compound or its precursors; second,forming a cabled intermediate from the strands by transposing them aboutthe longitudinal axis of the conductor at a preselected strand laypitch, and, texturing the strands in one or more steps including atleast one step involving application of a texturing process with aprimary component directed orthogonal to the widest longitudinalcross-section of the cabled intermediate, at least one such orthogonaltexturing step occurring subsequent to said strand transposition step.If a precursor to the desired superconducting compound remains, at leastone thermomechanical processing step at conditions sufficient to producephase transformation in the filament material is among in the texturingsteps. For materials requiring biaxial texture, such as members of theyttrium and thallium families of oxide superconductors, the texturingstep preferably includes application of a texturing process with aprimary component in a predetermined direction in the plane of thewidest longitudinal cross-section of the conductor.

The invention may be practiced with any form of composite strand, forexample a multifilamentary wire, monofilamentary wire or sandwichedlaminate. The stands may be prepared by any conventional method,including physical film forming methods such as sputtering or ion beamassisted deposition (IBAD), chemical film forming methods such aschemical vapor deposition (CVD), or the well-known powder-in-tube (PIT)process. In a preferred embodiment, the filament cross-section, filamenttwist pitch, and strand lay pitch are cooperatively selected to providea filament transposition area which is always at least ten times thepreferred direction area of a typical grain of the desired anisotropicsuperconducting compound at least ten times the preferred direction areaof a typical grain of the desired anisotropic superconducting compound.

To minimize the strain on the strand during the cabling operation, it ispreferred that low aspect ratio strands and relatively uncompactedfilament material be used during the strand transposition step, but thisis not necessary for the operation of the invention. Either partiallyaspecting the strand, partially pre texturing the filaments, or bothprior to strand transposition is within the scope of the invention.Precursors may also be advantageously be used instead of the desiredsuperconducting compounds to minimize strain damage during the cablingoperation, as they typically have much higher strain tolerances. By“precursor” is meant any material that can be converted to the desiredsuperconductor upon application of a suitable heat treatment. If anoxide superconductor is desired, for example, precursors may include anycombination of elements, metal salts, oxides, suboxides, oxidesuperconductors which are intermediate to the desired oxidesuperconductor, or other compounds which, when reacted in the presenceof oxygen in the stability field of a desired oxide superconductor,produces that superconductor. For example, there may be includedelements, salts or oxides of copper, bismuth, strontium, and calcium,and optionally lead, for the BSCCO family of oxide superconductors, or,as an example of an intermediate, BSCCO 2212 together withnon-superconducting phases which together are capable of being convertedto the desired oxide superconductor, BSCCO 2223. The formation of anintermediate may be desired in order to take advantage of desirableprocessing properties, for example, a micaceous structure or a highstrain tolerance, which may not be equally possessed by the desiredsuperconducting oxide. For example, uncompacted BSCCO 2223 precursorstypically have strain tolerances on the order of 20%, while stronglylinked BSCCO 2223 has a strain tolerance on the order of less than 1%.Precursors are included in amounts sufficient to form an oxidesuperconductor. In some embodiments, the precursor powders may beprovided in substantially stoichiometric proportion. In others, theremay be a stoichiometric excess or deficiency of any precursor toaccommodate the processing conditions used to form the desiredsuperconducting composite. For this purpose, excess or deficiency of aparticular precursor is defined by comparison to the ideal cationstoichiometry of the desired oxide superconductor. The addition ofdoping materials, including but not limited to the optional materialsidentified above, variations in proportions and such other variations inthe precursors of the desired superconducting oxides as are well knownin the art, are also within the scope and spirit of the invention.

The invention is scalable to large scale manufacturing techniques andhigh packing factor cable designs. Strands formed as described may becabled at high packing factors on conventional cabling equipment such asthat supplied by the Entwhistle Company of Hudson, Mass. Planetary orrigid cabling equipment may be used. A Rutherford-type cable ispreferred. This is a type of generally rectangular, compacted Litz cablewhose general assembly parameters are well known in the art. However,any type of cable, such as a partially transposed cable, or the Roebelor braided forms of Litz cable may be used. The strands may befabricated in accordance with the cabling parameters generally specifiedfor the particular piece of equipment. Typical parameters for aRutherford cabling machine are described in connection with the bismuthembodiment discussed below.

Processing to induce the desired texture may be done in one or moresteps and may include reaction methods, deformation methods, or othermethods such as magnetic alignment, depending on the texturingmechanisms most suited to the desired superconducting ceramic, but mustinclude at least one texturing step subsequent to strand transpositionwhich involves the application of a texturing process with a primarycomponent directed orthogonal to the widest longitudinal cross-sectionof the cabled intermediate, at conditions sufficient to induce at leastsubstantial c axis alignment of the grains in the filament. The primarycomponent may be either a component of a tensor, such as an appliedstrain, the gradient of a scalar, such as temperature, or the divergenceof a field, such as a magnetic field. Well-known techniques which aresuitable for orthogonal texturing include, for example, deformationtexturing for micaceous compounds such as BSCCO 2212 and 2223, magneticalignment for the BSCCO and YBCO families of compounds, andmelt-texturing via directional solidification for the YBCO family ofcompounds. For example, known techniques for texturing the two-layer andthree-layer phases of the bismuth-strontium calcium-copper-oxide familyof superconductors (BSCCO 2212 and BSCCO 2223, respectively) aredescribed in Tenbrink; Wilhelm, Heine and Krauth, Development ofTechnical High-Tc Superconductor Wires and Tapes, Paper MF-1, AppliedSuperconductivity Conference, Chicago(Aug. 23-28, 1992), and Motowidlo,Galinski, Hoehn, Jr. and Haldar, Mechanical and Electrical Properties ofBSCCO Multifilament Tape Conductors, paper presented at Materialsresearch Society Meeting, Apr. 12-15, 1993, and V. Chakrapani, D.Balkin, and P. McGinn, Applied Superconductivity, Vol. 1, No. 1/2, pages71-80, 1993. Multistep texturing processes, in which only some of thesteps meet the uniaxial orthogonal requirement, such as staged growthtexturing for YBCO and TBSCCO, may also be used. A staged growthtexturing technique for TBSCCO is described, for example, in co-pendingU.S. Ser. No. 08/147,061 filed Nov. 3, 1993 and entitled “Preparation ofHTSC Conductors by Deformation-Induced Texturing of SuperconductorsWithout Slip Systems”.

In a preferred embodiment, the desired anisotropic superconductingcompound is a micaceous or semi-micaceous superconducting oxide,preferably BSCCO 2212 or 2223. Cabled conductors comprising suchmicaceous compounds may be most successfully fabricated by transposingthe strands before full texture is developed in the strands and thentexturing the cabled strands in one or more steps including mechanicaldeformation by a non-axisymmetric technique at conditions sufficient toachieve a high aspect ratio in the strands, and texturing in thefilament material as further described below. In addition, becausemembers of the bismuth family tend to selectively form grains alignedwith the filament walls, the preferred inventive process includes one ormore additional requirements intended to overcome this tendency.

In one embodiment of the invention, a a magnetic alignment step with aprimary aligning force orthogonal to the widest longitudinalcross-section of the cabled intermediate may be included after cablingin addition to the deformation step in order to reorient the grains nearthe cross-over points in the desired direction. Recently, a magneticalignment technique has demonstrated good texture in 2212 thick filmmaterial on Ag [H. B. Liu and J. B. Vander Sande, submitted to PhysicaC, (1995)] A 2212 melt-growth heat treatment performed in a homogeneousmagnetic field of 2-10 T at temperatures of 820 to 840C produced texturewith the orientation of the c direction of the 2212 grains parallel tothe magnetic field.

In another embodiment of the invention, the filament dimensions,filament twist pitch and strand lay pitch are cooperatively selected sothat the filament width in the direction orthogonal to the c-c′ axis isalways greater than, and preferably at least twice as great as thefilament height in the direction parallel to the c-c′ axis. This latterrequirement can be met by selecting an aspected filament cross-section,a strand lay pitch which is not an even multiple of the filament twistpitch and avoiding pitches which tend to align the filament wallsparallel the c-c′ axis. Grain growth perpendicular to the desireddirection will be less pronounced with this aspected, angled filamentlayout.

The strands may be made by any well-known method, using, for example,either oxide or metallic precursors. However, multifilamentary wires andtapes made by the PIT process are preferred. The general PIT process isdescribed, for example, in U.S. Pat. Nos. 4,826,808, and 5,189,009 toYurek et al. and W. Gao & J Vander Sande, Superconducting Science andTechnology, Vol. 5, pp. 318-326, 1992, which teach the use of a metalalloy precursor having the same metal content as the desiredsuperconducting oxide, and in C. H. Rosner, M. S. Walker, P. Haldar, andL. R. Motowido, “Status of HTS superconductors: Progress in improvingtransport critical current densities in HTS Bi-2223 tapes and coils”(presented at conference ‘Critical Currents in High Tc Superconductors’,Vienna, Austria, April, 1992) and K. Sandhage, G. N. Riley Jr.,. and W.L. Carter, “Critical Issues in the OPIT Processing of High Jc BSCCOSuperconductors”, Journal of Metals, 43,21,19, which teach the use ofeither a mixture of powders of the oxide components of thesuperconductor or of a powder having the nominal composition of thesuperconductor, all of which are herein incorporated by reference.Generally speaking, the PIT process for making multifilamentarysuperconducting composite conductors includes the stages of forming apowder of superconductor precursor material, loading this powder intonoble metal containers and deformation processing one or more filledcontainers by a longitudinal reduction technique to provide a compositeof reduced cross-section including one or more filaments of precursormaterial in intimate contact with a surrounding noble metal matrix.Multifilamentary composite conductors undergo rebundling operations atone or more points during the precursor fabrication stage. Thisoperation involves assembling filled containers in some close packed orother symmetric arrangement, possibly around a hollow ornon-superconducting central supporting core, inside a metal tube, orboth, followed by further longitudinal reductions. Utilizing a planetaryset-up, a strand with an aspect ratio as high as 2:1 may used to makeRutherford cable. An intermediate comprising BSCCO 2223 or itsprecursors will, however, require post-cabling deformation to an averagestrand aspect ratio of 3:1 or greater to create adequate texturing forcommercially acceptable current-carrying capacity.

In accordance with a preferred embodiment of the invention, thetransposition area is selected to be at least ten times, and preferablyat least thirty times, the preferred direction area of a typicalsuperconducting grain to permit crystallographic grain alignment in thedesired direction at the filament cross-over points.

In accordance with the invention, the strands 110 to be transposed arespooled in equal amounts onto N spools 510, where N is the number ofstrands to be included in the intermediate 120. These spools are loadedonto the cabling machine 500, shown schematically in FIG. 5. Each spoolhas an independent tensioning device to provide uniform tension controlon pay-off. The applied strand tension is preferably less than 0.2 ofthe tensile strength of the strand. The spools rotate together about acommon rotation axis 520. In the machine shown in FIG. 5, a planetarycontrol provides the capability to rotate the spool through its centroidabout an axis parallel to the rotation axis. In this configuration, thesame side of the stand always faces the same direction in the cable.However, the invention may also be practiced on rigid cabling machines,which do not provide this capability, without adversely affecting thedesired texturing of the superconducting cable.

Each of the spools pays off to a “gathering point” at a fixed positionfrom the mandrel 530 and approximately circumferentially symmetric aboutthe mandrel. The mandrel is a spade-shaped tooling that is non-rotatingand located on the common rotation axis. The strands wrap around themandrel and pay-off into a shaping turks-head roll 540 that defines thecable width and thickness. The rate that the cable is pulled through theturks-head relative to the rotation rate around the common axis definesthe cable lay pitch. These parameters are not independent in anoptimized intermediate cable, one that is robust for handling and postcabling deformation. The thickness t of the intermediate cable shouldtypically be chosen to be not more than 1.8 times the strand diameter d,to “lock” the strands together by “upsetting” them. Locking is typicallydone by a slight deformation which is sufficient to change the shape ofthe strand from round to elliptical but not substantially change itscross-sectional area. The width w of the intermediate cable shouldtypically be chosen to be not significantly more than the value ofN/2*d, to provide “locking” in the width direction. The lay pitch shouldtypically be chosen to be about n*d*N, where n is a constantcharacteristic of the cabling equipment which is typically in the rangeof 3 to 6, and most typically in the range of 3.7 to 5.3. Lay pitchesbelow this range will result in excessive compaction and cablingdifficulties. Cables with significantly longer lay pitches can becomemechanically unstable. The strands can be pulled through the turks-headwith a capstan (a rotating wheel), or by a caterpuller (between twoparallel belts). Either of these may be assisted by a powered turks-headroll replacing a standard non-powered turks-head driven by a torquesomewhat less than that required to pull the cable through the rolls.The intermediate cable is taken-up onto a spool under a tensionpreferably on the order of N* applied strand tension at the spools.

To fully texture the intermediate and form a superconducting cable, theintermediate is mechanically deformed in one or more steps by anon-axisymmetric technique, preferably at conditions sufficient toachieve a high aspect ratio and a packing factor of at least 75% andpreferably at least 85%, and to texture the filament material. The postcable deformation is accomplished by rolling or additional turks-headrolling. Cold rolling or powered turks heading is preferred. It ispreferred that a total strain of up to 90% be applied in 1-25 passes.Intermediate anneals may be performed to reduce strain hardening of thematrix material. Tension is typically applied on the pay-off and take-upside of either of these rolling operations. The tension controls affectthe neutral point in the rolling operation and are typically chosen tobe less than half of the yield strength of the composite. When poweredturks-heads are used, it is possible to combine the cable fabricationwith substantial deformation. This is not otherwise possible in astandard pull-through turks-head, because of the limited pull strengthof the cable on the exit side. When a powered turks head is used, theexiting cable thickness can be up to 80% or less than of the 1.8*dguideline mentioned above.

Thereafter the intermediate cable is further thermomechanicallyprocessed in one or more steps in an oxidizing atmosphere at conditionssufficient to produce at least one of the effects of texturing, and, ifa precursor to the desired micaceous superconducting oxide remains,phase transformation in the filament material, thereby forming asuperconducting cabled conductor from the intermediate.

In the preferred embodiment, a final heat treatment is performed underconditions suitable for healing strain-induced cracks in the filamentmaterial. For most oxide superconducting composites, the criticalcurrent is independent of the amount of tensile strain placed on thecomposite until the strain reaches a threshold value, commonly referredto as the critical strain of the material. Above that threshold, thecritical current value decreases asymptotically with increasing tensilestrain due to formation of localized microcracks in the filamentmaterial. A melt-textured growth technique such as that described inKase et al, IEEE Trans. Mag. 27(2), 1254(1991) may be used forcrack-healing in BSCCO 2212. Suitable final heat treatment processes forBSCCO 2223 are described, for example, in copending applications U.S.Ser. No. 08/041,822 filed Apr. 1, 1993 and entitled “Improved Processingfor Oxide Superconductors”, U.S. Ser. No. 08/198,912, filed Feb. 17,1994 and also entitled “Improved Processing for Oxide Superconductors”,and in U.S. Ser. No. 08/553,184, filed of even date herewith andentitled “Processing of Oxide Superconducting Cables”. If the localtensile strain is much greater than the critical strain value,micro-crack formation can occur to such an extent that healing duringthermomechanical processing becomes impossible. Thus, it is preferredthat the maximum strand bend radius formed in the intermediate be lessthan about 8%.

The invention provides a cabled conductor comprising a plurality oftransposed strands each comprising one or more preferably twistedfilaments comprising textured anisotropic superconducting compoundswhich have substantial crystallographic grain alignment that isdirectionally independent of the rotational orientation of the strandsand filaments in the cabled conductor. The anisotropic superconductingcompounds are textured such that the crystallographic c axis grainalignment of each grain of the superconducting compound is substantiallyunidirectional and perpendicular to the longitudinal axis, andpreferably to the widest longitudinal cross-section, of the cabledconductor independent of the rotational orientation of the strands andfilaments in the cabled conductor.

Because the low density powder which is included in the strands at thetime they are transposed has a much higher strain tolerance than thecompressed and textured superconducting ceramic material, the crackingcharacteristic of most superconducting ceramics is reduced and cableswith packing factors in excess of 75% may be manufactured in accordancewith the invention.

The invention may be further understood from the following examples:

EXAMPLE 1

A 91 filament composite was made by the PIT process with anapproximately a hexagonal array filament pattern using standardmonofilament 2223 precursor in a fine Ag sheath. Precursor powders wereprepared from the solid state reaction of freeze-dried precursors of theappropriate metal nitrates having the nominal composition of1.8:0.3:1.9:2.0:3.1 (Bi:Pb:Sr:Ca:Cu)?]. Bi₂O₃, CaCO₃, SrCO₃, Pb₃O₄-, andCuO powders could equally be used. After thoroughly mixing the powdersin the appropriate ratio, a multistep treatment (typically 3-4 steps) ofcalcination (800° C.±10° C., for a total of 15 h) and intermediategrinding was performed in order to remove residual carbon, homogenizethe material and generate a BSCCO 2212 oxide superconductor phase. Thepowders were packed into silver sheaths to form a billet. The billetswere extruded to a diameter of about ½ inch (1.27 cm) and annealed at450 C for 1 hour. The billet diameter was narrowed with multiple diesteps, with a final step drawn through a hexagonally shaped die into asilver/precursor hexagonal monofilament wires.

Eighty-nine wires 0.049×0.090″, one 0.1318 round and one 0.055 roundwires were assembled and inserted into a 0.840″ outer diameter by 0.740″inner diameter silver tube to form a bundle. The assembly was baked forfour hours at 450 degrees the bundle was allowed to cool and then drawnthrough to 0.072 via successive 20% and 10% pass reductions to for amulti-filamentary round strand. At 0.072″ it was annealed at 450 degreesfor one hour, allowed to cool and drawn to 0.354″ It was again annealedat 450 degrees C. for one hour, allowed to cool and then drawn to 0.245″diameter. The composite was annealed in air at 300C for nominally 10minutes. The material was divided approximately equally into 8 parts andeach was layer wound onto a cabling spool.

An 8 strand Rutherford cable was made from 91 filament composite strand.A rigid cabling configuration was used, where the spools' orientationare fixed relative to the rotating support that holds them. The tensionon each strand was controlled by magnetic breaks and set to nominally0.5 inch-pounds. The width and thickness of the cable were set by anon-powered turks-head to be 0.096 and 0.048 inch, respectively. Thecable lay pitch was set by a capstan take-up speed relative to therotations speed to be nominally 1.03 inch. After cabling, the materialwas heat treated at 760 C for 2 hr. in 0.1 atm of oxygen. The cable wasthen rolled to at thickness of 0.0157 inch and heat treated for 6 hr. at827 C in 7.5% oxygen in nitrogen atmosphere. The cable was finally turkshead rolled to 0.0126 inch in thickness. A final heat treatment of 40hr. at 827 C, 30 hr. at 808 C, and 30 hr. at 748 C, all in 0.075 atm ofoxygen in nitrogen was employed. The Je at 77K (B=0) was 2996 A/cm² at afill factor of nominally 25% superconductor cross section. Thevoltage/current characteristics of the sample in 0 magnetic field areshown in Exhibit 1.

EXAMPLE 2

A 91 filament composite was made with an approximately a hexagonal arrayfilament pattern as described in Example 1 above. In this example, themultifilament composite was further drawn to nominally 0.028 inchdiameter and turk-headed or drawn through a square die to 0.0245 inch ona side. The square cross section composite was annealed in air at 300Cfor nominally 10 minutes. The material was divided approximately equallyinto 8 parts and each was layer wound onto a cabling spool.

An 8 strand Rutherford cable is made from 91 filament composite strand.A “ferris wheel” cabling configuration is used, where the spools'orientation in space is fixed as it rotates around the axis of thecabler, similar to a seat on a ferris wheel. The tension on each strandis controlled by magnetic breaks and set to nominally 0.5 inch-pounds.The width and thickness of the cable were set by a non-poweredturks-head to be 0.096 and 0.048 inch, respectively. The strands enterthe turks-head with the sides of the square cross section maintainedparallel to the sides of the resulting rectangular cable. The cable laypitch is set by a capstan take-up speed relative to the rotations speedto be nominally 1.03 inch. After cabling, the material is heat treatedat 760 C for 2 hr. in 0.1 atm of oxygen. The cable is then rolled to atthickness of 0.0157 inch in a single pass. The cable is then heattreated for 6 hr. at 827 C in 7.5% oxygen in nitrogen atmosphere. Thecable is finally rolled to 0.0145 inch in thickness in a single pass. Afinal heat treatment of 40 hr. at 827 C, 30 hr. at 808 C, and 30 hr. at748 C, all in 0.075 atm of oxygen in nitrogen is employed. The Je at 77K=0) is 2280 A/cm² at a fill factor of nominally 20% superconductor crosssection.

The various features and advantages of the invention may be seen fromthe foregoing description and examples. Iterative variations on theprocesses described above, such as changes in the materials, the numberand type of texturing steps, and the cabling styles and equipment usedwill be seen to be within the scope of the invention. Many modificationsand variations in the preferred embodiments illustrated will undoubtedlyoccur to those versed in the art, as will various other features andadvantages not specifically enumerated, all of which may be achievedwithout departing from the spirit and scope of the invention as definedby the following claims.

1. A cabled conductor comprising a plurality of transposed strands eachcomprising one or more filaments comprising grains of texturedanisotropic superconducting compounds which have crystallographic grainalignment that is substantially unidirectional and directionallyindependent of the rotational orientation of the strands and filamentsin the cabled conductor.
 2. A cabled conductor according to claim 1wherein each strand has a preselected strand lay pitch and each filamenthas a preselected filament cross-section and filament twist pitch, andthe strand lay pitch, filament cross-section and filament twist pitchbeing cooperatively selected to provide a filament transposition areapermitting the crystallographic grain alignment in the grain directionat the filament cross-over points.
 3. A cabled conductor according toclaim 1, wherein each strand has a strand lay pitch and each filamenthas a filament cross-section and filament twist pitch, and the filamentcross-section, filament twist pitch, and strand lay pitch arecooperatively selected so that the filament width in the plane of thewidest longitudinal cross-section of the conductor is greater than thefilament height of the widest longitudinal cross-section of theconductor.
 4. A cabled conductor according to claim 1, wherein thecabled conductor has an aspect ratio, width to height of the conductor,greater than or equal to about 3:1.
 5. A cabled conductor according toclaim 1, wherein the cabled conductor has an aspect ratio, width toheight of the conductor, greater than or equal to about 5:1.
 6. A cabledconductor according to claim 1, wherein the cabled conductor has apacking factor of at least about 75 percent.
 7. A cabled conductoraccording to claim 1, wherein the cabled conductor has a packing factorof at least about 85 percent.
 8. A cabled conductor according to claim 2wherein the strand lay pitch, filament cross-section and filament twistpitch are cooperatively selected to provide a filament transpositionarea which is always at least ten times the preferred direction area ofa typical grain of the desired anisotropic superconducting compound. 9.A cabled conductor according to claim 8 wherein the strand lay pitch,filament cross-section and filament twist pitch are cooperativelyselected to provide a filament transposition area which is always atleast thirty times the preferred direction area of a typical grain ofthe desired anisotropic superconducting compound.
 10. A cabled conductorcomprising a plurality of strands transposed about the longitudinal axisof the conductor, each strand comprising one or more filamentscomprising grains of an anisotropic superconducting compound texturedsuch that the crystallographic c axis alignment of each grain of thesuperconducting compound is substantially perpendicular to thelongitudinal axis of the cabled conductor, independent of the rotationalorientation of the strands and filaments in the cabled conductor.
 11. Acabled conductor according to claim 10 wherein each strand furthercomprises a conductive matrix material surrounding or supporting thefilaments.
 12. A cabled conductor according to claim 10 wherein eachstrand is insulated.
 13. A cabled conductor according to claim 10,wherein each strand has a strand lay pitch and each filament has afilament cross-section and filament twist pitch, and the filamentcross-section, filament twist pitch, and strand lay pitch arecooperatively selected so that the filament width in the plane of thewidest longitudinal cross-section of the conductor is greater than thefilament height of the widest longitudinal cross-section of theconductor.
 14. A cabled conductor according to claim 10, wherein thecabled conductor has an aspect ratio, width to height of the conductor,greater than or equal to about 3:1.
 15. A cabled conductor according toclaim 10, wherein the cabled conductor has an aspect ratio, width toheight of the conductor, greater than or equal to about 5:1.
 16. Acabled conductor according to claim 10, wherein the cabled conductor hasa packing factor of at least about 75 percent.
 17. A cabled conductoraccording to claim 10, wherein the cabled conductor has a packing factorof at least about 85 percent.
 18. A cabled conductor according to claim10 wherein the anisotropic superconducting compound is a superconductingceramic.
 19. A cabled conductor according to claim 18 wherein thesuperconducting ceramic material comprises a superconducting oxide. 20.A cabled conductor according to claim 19 wherein the superconductingceramic is micaceous or semi-micaceous.
 21. A cabled conductor accordingto claim 20 wherein the superconducting ceramic is a member of thethallium family of superconducting oxides.
 22. A cabled conductoraccording to claim 20 wherein the superconducting ceramic is a member ofthe bismuth family of superconducting oxides.
 23. A cabled conductoraccording to claim 22 wherein the filaments are twisted and the filamentcross-section, filament twist pitch, and strand lay pitch arecooperatively selected so that at each point on the filament, regardlessof how it is twisted, the filament width in the plane of the widestlongitudinal cross-section of the conductor is always greater than, andpreferably twice as large as the filament height orthogonal to thewidest longitudinal cross-section of the conductor.
 24. A cabledconductor according to claim 22 wherein the superconducting ceramic isBSCCO
 2212. 25. A cabled conductor according to claim 22 wherein thesuperconducting ceramic is BSCCO
 2223. 26. A cabled conductor accordingto claim 19 wherein the superconducting ceramic is a member of the rareearth family of superconducting oxides.
 27. A cabled conductor accordingto claim 26 wherein the cabled conductor is a Litz cable.
 28. A cabledconductor according to claim 27 wherein the cable is a Rutherford cable.29. A cabled conductor according to claim 27 wherein the cable is aRoebel cable.
 30. A cabled conductor according to claim 27 wherein thecable is a braided cable.
 31. A cabled conductor according to claim 10wherein each filament is twisted.
 32. A cabled conductor according toclaim 31 wherein each strand has a preselected strand lay pitch and eachfilament has a preselected filament cross-section and filament twistpitch, and the strand lay pitch, filament cross-section and filamenttwist pitch being cooperatively selected to provide a filamenttransposition area permitting crystallographic grain alignment in thegrain direction at the filament cross-over points.
 33. A cabledconductor according to claim 32 wherein the strand lay pitch, filamentcross-section and filament twist pitch are cooperatively selected toprovide a filament transposition area which is always at least ten timesthe preferred direction area of a typical grain of the desiredanisotropic superconducting compound.
 34. A cabled conductor accordingto claim 33 wherein the strand lay pitch, filament cross-section andfilament twist pitch are cooperatively selected to provide a filamenttransposition area which is always at least thirty times the preferreddirection area of a typical grain of the desired anisotropicsuperconducting compound.